1. Field of the Invention
The present invention relates generally to gamma scanning apparatus that measures radiation emitted from a fuel assembly at a nuclear power plant and thus measures burn-up and a power distribution of the fuel assembly. More particularly, the invention is directed to gamma scanning apparatus suitable for measuring burn-up and a power distribution of a fuel assembly accurately, not depend on the accuracy of an installation location and installation angle of the fuel assembly, and hence for measuring radiation emissions from a number of fuel assemblies within a short time.
2. Description of the Related Art
It is traditionally known that gamma rays are used to measure the burn-up of a spent fuel assembly, as in JP-10-332873-A.
A method of measuring burn-up and a power distribution by measuring the radiation from an active fuel assembly at a nuclear power plant is also coming into use in recent years. In these measurements, a housing is fixedly installed to obtain a constant relative angle between the housing and an outer surface of the fuel assembly having a square cross section, and the fuel assembly is moved vertically with the relative angle kept constant. In this case, a collimator is provided on a radiation detector present inside the housing, and a central portion of the height section of the fuel assembly that is included in a solid angle defined by the collimator is identified as a vertical central position. The measurement of the vertical burn-up and power distribution of the fuel assembly is conducted by repeating several steps. First, the fuel assembly is moved vertically and fixed to a given position. Next, a first measuring operation is performed and then the fuel assembly is moved vertically once again. After this, the fuel assembly is fixed again and a second measuring operation follows. For measurement at a relative angle between at least two positions, the burn-up and the power distribution have traditionally been measured in a vertical direction with the angle kept constant, then this angle has been changed and the fuel assembly fixed, and the angle has been kept constant once again and the fuel assembly moved vertically again for measurement. In addition, referring to measuring angles, if the angle at which one side of the square of the fuel assembly having the square cross section faces the detector is defined as 0 degrees, for example, measurements have been conducted at an angle of 45 degrees and the angle of 225 degrees established by rotating the fuel assembly through 180 degrees from the state of 45 degrees.
Given a constant gamma-ray intensity distribution in the fuel assembly, if the relative angle between the fuel assembly and the housing changes, the self-shielding effect of the fuel assembly and other factors change a counting rate of the radiation detector within the housing. Before starting the measurements, therefore, it has been necessary to accurately determine an axial rotational position of the fuel assembly with respect to the housing.